When a non-polar liquid which has a low dielectric constant flows past an insulator an electrostatic charge can build up on the insulator. This electrostatic charge can result in sparks and partial discharges which break down or pit the insulation. If the flowing liquid is combustible, such as an aircraft fuel, the sparks may ignite the liquid and cause an explosion.
The tendency for liquid to generate, electrostatic charges is called the charging tendency, which is equal to the charge density, (q/v), which equals the integral of (idt/v) where q is the charge, d is the volume of the liquid, i is the charging current, and t is flow time. If i is taken to be the average current, then the charge density reduces to (it/v) which equals the charging current over the flow rate, where (v/t) is the flow rate.
The electrostatic charges on insulators in contact with a moving liquid can be reduced to some extent by grounding conductors in contact with the liquid or the insulators. However, this is not always very effective.
The charging tendency of a liquid can be reduced by adding anti-static additives to the liquid which make the liquid conductive. Obviously, these additives cannot be used if the liquid is to function as an insulator.
Finally, some reduction in the charging tendency of a liquid can be obtained by filtering the liquid through clay. While this method was used at one time to continuously filter transformer oil, it since has been dropped because the clay is not very effective and is quickly exhausted. Also, the clay may remove aromatics from the oil, which are desirable because they tend to reduce corona.